This says any one to six digit string after the hostname will be passed to
id-shorts.cgi. Of course, you'll want to change the path to match the actual
location of your MT install and make the sure the matched text doesn't conflict
with a file or another rewrite rule. After this, you should be able to visit
http://super-awesome-url.biz/[mt:EntryID] and go to the entry.

Tags

This plugin adds the <mt:entryshorturl> tag. The output of this tag is based
on the blog level plugin setting for the short url template, which defaults to
<mt:blogurl><mt:entryid>.